The present invention relates to a process for the purification of crude chlorophosphazene. More particularly, the present invention relates to a chlorophosphazene purification process involving an intermediate purification or partial purification stage in which the crude chlorophosphazene is subjected to at least two water-washings, following which a final purification step is performed by a conventional chlorophosphazene purification procedure, such as recrystallization, distillation, sublimation or melt-filtering and a substantially purified chlorophosphazene is recovered.
Heretofore, in the production of chlorophosphazenes or phosphonitrilic chlorides, purity of the compounds generally was not a factor since their specific uses such as for flame retardants and thermally resistant resins did not require removal of contaminates. Generally, chlorophosphazenes (NPCl.sub.2).sub.x where x equals 3 through 9 are largely produced by the reaction of phosphorus pentachloride and ammonium chloride which results in compounds, that is, x=3-9, having a high degree of impurities. Although several methods of purification are available, a good degree of purification to date has been difficult to achieve. Thus, chlorophosphazenes which are contaminated with impurities, upon heating in excess of 250.degree. C. will often result in gelled polymers which are benzene insoluble.
The general approach of prior art methods of purification has been the separation of isomers of chlorophosphazene, either by utilization of the differences of boiling points of the isomers or by the different reactivities of the isomers with aqueous bases. Specific purification methods have involved the extraction of chlorophosphazene from petroleum ether solutions with sulfuric acid, U.S. Pat. No. 3,008,799; controlled crystallizaton in a variety of solvents, U.S. Pat. No. 3,378,353; separation of the trimer and tetramer from the produced mixtures through distillation involving a spinning band column, U.S. Pat. No. 3,379,510; contacting molten chlorophosphazenes with an inert solvent vapor so as to selectively vaporize the cyclic trimer, separating a solvent vapor phase laden with trimer and some tetramer from the molten residue, condensing it to form a solution of trimer and tetramer in the solvent and subsequently recovering trimer together with some tetramer from the solution; U.S. Pat. No. 3,677,720; steam distillation of chlorophosphazenes resulting in hydrolysis of x=4-9 and hence separation of the trimer, Chemical Abstracts, Volume 77, Page 540, 159648D (1972); saponification and hydrolysis of chlorophosphazenes by treatment with aqueous sodium or ammonium hydroxide resulting in unreacted trimer and tetramer, U.S. Pat. No. 3,694,171; and, contacting a crude chlorophosphazene with a Bronsted base, removing water formed from the reaction of the Bronsted base with the impurities in the chlorophosphazene, and then recovering at least a very high purity cyclic chlorophosphazene trimer, U.S. Pat. No. 3,952,086.
As a general rule, prior art methods of purification have avoided the use of water in the purification of crude chlorophosphazenes in view of the well known instability of such chlorophosphazenes toward protonic molecules such as water, methanol, ethanol, isopropanol, alkyl amines and other basic materials. Thus, the use of these protonic materials including water, has been generally avoided in the prior art methods of purification since these materials often cause displacement or replacement of the covalent and/or ionic chloride atoms on the phosphorus atoms of the chlorophosphazene and lead to attendent problems in polymerization and derivatization of the chlorophosphazene.
Of the prior art methods of purification listed above, the chemical abstracts article which employs steam distillation and U.S. Pat. No. 3,694,171 which employs aqueous sodium or ammonium hydroxide in the purification process appear to represent departures from the general rule. However, the purification processes disclosed in these references have been found to suffer from very serious disadvantages which virtually eliminates or at least materially limits their usefulness. Thus, it has been found that neither of the processes disclosed in the aforementioned references provide for the production of chlorophosphazenes of sufficient purity to permit the preparation of polydichlorophosphazene polymers without also producing appreciable amounts of gel.